The present invention relates to a method of producing a hot dip aluminum coating steel sheet (i.e. hot dip aluminizing steel sheet of high quality.
Hot dip aluminum coating steel sheet generally exhibits a high resistance to heat and, due to this fact, finds various uses such as the material of exhaust pipes of automotive engines, material of heating instruments for household uses, and so forth. In recent years, however, the materials of the exhaust pipes of automotive engines are required to withstand higher temperature. In such uses at high temperature, any coating defect such as imperfect coating, pin hole or the like causes a rapid corrosion of the base iron exposed through such coating defect. For this reason, there is an increasing demand for hot dip aluminum coating steel sheets having no coating defects such as imperfect coating and pin holes. The material of parts used in the exhaust systems of automotive engines is required to have also an excellent oxidation resisting property at high temperature. To this end, it is necessary that the aluminum coating layer is rapidly diffused into the base iron by the heat during the use so as to form an Fe-Al diffused alloy layer having excellent oxidation resisting property, in addition to the elimination of the coating defects mentioned before.
According to the specification of U.S. Pat. No. 2437919, the occurrence of the coating defect such as imperfect coating and pin holes in the actual hot dip aluminum coating process is attributable to the existence of nitrogen, a small amount of oxygen and/or moisture included in gas of reducing atmosphere, which nitrogen, oxygen and moisture form nitrides, oxides and hydrides which a float as scums on the surface of the coating bath in a snout. It is said that the insufficient coating and pin holes are caused by deposition of the scum on the surface of the strip running through the snout.
The following counter-measures have been taken in order to prevent the occurrence of coating defect attributable to the deposition of the scum:
(1) To avoid generation of scum; PA1 (2) To change the nature of the scum such that the scum does not attach to the strip or that the Fe-Al diffusion reaction can be made satisfactorily through the deposited scum; and PA1 (3) To mechanically remove the scum from the strip in the molten aluminum bath.
The generation of scums can be avoided by preventing the moisture and oxygen in the reducing atmosphere from coming into the snout. In recent years, it is not so difficult to industrially attain a reducing atmosphere having an O.sub.2 concentration of 5 to 6 ppm or lower and a dew point not higher than -40.degree. C., because of the use of nonoxidizing furnace which permits to maintain higher pressure in the furnace. Such low oxygen content and low moisture content appreciably contribute to the prevention of insufficient coating, but this countermeasure solely cannot prevent the occurrence of the coating defect perfectly. Another known method for preventing generation of scums is to dispose a bath of lead or bismuth between the molten aluminum bath and the reducing gas atmosphere in the snout. This method, however, involves a problem in that the heat resisting property and the corrosion resisting property of the hot dip aluminum coating steel sheet are decreased undesirably by the lead and bismuth and, therefore, has not been carried out industrially.
As an example of the second countermeasure which intends to convert the nature of the scum, the specification of the U.S. Pat. No. 2437919 discloses a method in which sodium vapor is introduced into the snout to form powdered sodium aluminate (AlNaO.sub.2) on the surface of the coating bath. The sodium aluminate formed on the surface of the coating bath in the snout does not attach to the strip and suppresses the generation of scums which are formed through mutual reaction between the coating bath and the protecting atmosphere. This countermeasure, however, suffers also from the following disadvantage. Namely, the although advantageous effect of addition of the sodium vapor is remarkable when the dew point of the atmosphere is between 30.degree. and -20.degree. C., it is impossible to perfectly prevent the occurrence of coating defects. Further, its effect becomes not appreciable when the dew point is below -40.degree. C. In addition, the sodium vapor introduced into the snout portion considerably deteriorates the coating adhesion of the hot dip aluminum coating steel sheet. This undesirably increases the tendency of separation of the coating layer during a press work which may be conducted subsequently to the coating. Consequently, the hot dip aluminum coating steel sheet cannot withstand the severe condition of press work.
The countermeasure comprising the step of mechanically wiping off the scums from the strip while the strip is in the aluminum bath is quite effective in eliminating the coating defect, but suffers a problem in that scratches caused in the surface of the strip while the latter is in the aluminum bath remain in the coated product to degrade the appearance of the coated product. Such scratches also tend to allow separation of the coated layer when the coated structure is worked by, for example, a press. This method, therefore, has not been successfully carried out in an industrial scale.
The resistance of the aluminum-coated steel sheet to high temperature exceeding 700.degree. C. is largely affected by the components of the steel used as the base sheet to be coated. For instance, in case of a rimmed steel or aluminum-killed steel, the base iron is liable to be oxidized because of cracking in the alloy layer caused during coating or skin-passing. Consequently, the oxidation resistance of the product of such steels is impaired seriously. To avoid this problem, Japanese Patent Publication No. 15454/1978, which claims a convention priority on U.S. Pat. No. 205569, proposes a steel in which Ti content is 4 to 10 times as large as the C content. The current demand for the excellent heat resisting property, however, cannot be met even by this method.
In recent years, in addition to the oxidation resisting property at high temperature above 700.degree. C., there are also demand for superior high-temperature strength and fatigue strength. These requirements are met by adding to the steel some alloying elements which generally serve to impede the hot dip aluminum coating to degrade the quality of the product.